A traditional camera module includes an image sensor and one or more lens set. The lens set is disposed above the image sensor for the incident ray image mapping to the image sensor. Camera module with the image sensor can be applied to digital cameras, digital video recorders, mobile phones, smart phones, monitors and others electronic products with camera function. For camera module, it needs to ensure the image quality and meet the requirement of trend of miniaturization of the electronic products.
For camera module, focusing is a critical step to ensure the recorded images with a good image quality. Focusing is by adjusting the distance between the lens and the image sensor such that the image plane falls on the imaging surface of the image sensor. Currently, the common camera module uses an automatically focusing, i.e., in accordance with the distance from the captured object, by a driving circuit to drive the motor for pushing the lens to a corresponding position, so that the image of the captured object has a good resolution.
Automatically focusing includes an active autofocus and a passive autofocus. The active autofocus is utilizing by a camera module emitting infrared or ultrasonic to the captured object, and then the reflected infrared or ultrasonic back to a sensor and computed by a digital signal processor to obtain the distance between the camera mode and the captured object, followed by controlling an actuator to move the lens to achieve accurately focusing. The passive autofocus is utilizing a camera module to receive the reflective light from the captured object to determine the distance for automatically focusing.
For camera module, in order to ensure the image quality, it needs to strictly control the factors of affecting imaging quality to meet the requirement of higher and higher resolution of the lens module. For camera module assembly process, the tolerance of the lens and the actuator will cause the offset of the modulation transfer function (MTF) curve, so that the MTF value at the center and the corner of the lens produces a balanced condition, and therefore impacting automatic focus and imaging quality of the camera module. Characteristic diagram of MTF reflects imaging quality performance at the position from the central region to the corner (edge) of the lens.
Therefore, based-on the shortcomings of prior arts, the present invention provide a newly method for automatically focusing applied to a camera module to enhance the quality of the camera module.